Cash Settlements and Valuation Gains and Losses on Commodity Derivative Contracts and Interest Rate Swaps (Parenthetical) (Detail) (Commodity derivatives, USD $)	12 Months Ended
	Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009
Derivative Instruments, Gain (Loss) [Line Items]
Realized (gain) loss on derivative contracts	$ (50,713,000)	[1]	$ 224,337,000	[1]	$ 348,022,000	[1]
Out-of-period settlements
Derivative Instruments, Gain (Loss) [Line Items]
Realized (gain) loss on derivative contracts	(48,100,000)
Realized (gain) on derivative contracts	(111,000,000)		(114,400,000)
Realized loss on derivative contracts	$ 62,900,000

[1] Includes $48.1 million net realized gains ($111.0 million realized gains and $62.9 million realized losses) for the year ended December 31, 2011 related to settlements of commodity derivative contracts with contractual maturities after the quarterly period in which they were settled ("out-of-period settlements"). Includes $114.4 million of realized gains for the year ended December 31, 2010, related to out-of-period settlements. There were no out-of-period settlements during 2009.